Biasing arrangement for linear detector array

ABSTRACT

A detector includes a plurality of detector elements in a linear array. A bias source is connected to the input terminal of each detector element. The bias source includes a first bias connected to the input terminals of a first set of detector elements including alternating detector elements and is also connected through an associated measurement device to the output terminals of a second set of detector elements including the remaining detector elements. The bias source also includes a second bias, different from the first bias, and connected through an associated measurement device to the output terminals of the first set of detector elements. The second bias is also connected to the input terminals of the second set of detector elements.

This application claims priority from United States Provisional patentapplication Ser. No. 60/002,347, filed on Aug. 15, 1995, entitled "NovelMethod of Biasing Photoconductive Linear Detector Array."

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to detectors and, more particularly, to thebiasing of a linear array of photoconductive detectors.

2. Background Art

The use of photoconductive detectors for measuring radiation iswell-known. Because of their high sensitivity, photoconductive detectorssuch as PbS and PbSe are particularly effective in measuring infraredradiation. Detection of infrared radiation is used by the military fortracking warm vehicles and in night vision devices, is used by medicalinstrument manufacturers for measuring glucose and other bodyconstituents in a noninvasive manner and is used by scientificinstrument manufacturers for measuring chemical composition andstructure.

In general, the resistance of the photoconductive detector changes whenthe radiation falls on its surface. Resistance changes can be measuredas an electrical signal change and the intensity of the detectedradiation can be estimated by the magnitude of resistance change.

Photoconductive detectors typically require a bias current or voltage tooperate, such as a direct current bias. The sensitivity of the detectoris proportional to the magnitude of the applied bias. It is preferred tosupply a high bias to such a detector to increase its sensitivity and toovercome the noise of the electronics associated with the detector in anoverall detection system.

The bias can be delivered to the detector in various manners, includinga voltage divider, a constant current or a constant voltage. FIG. 1Ashows a known voltage divider arrangement in which a detector 2 isattached at one end to a bias electrode 4 and is connected at its otherend to a load resistor 6 which is grounded. The voltage across the loadresistor 6 is supplied through capacitor 8 to amplifier 10 whichgenerates an output signal related to the intensity of the radiation 12impinging upon the detector 2. In the voltage divider arrangement, theincident radiation 12 modulates the resistance of the detector 2 and,consequently, changes the current flowing through the detector 2 and theload resistor 6. These current changes are converted to voltage changesacross the load resistor 6. The AC components of the voltage changes arepassed through capacitor 8 to the amplifier 10 for further processing.

FIG. 1B shows a constant current mode of operation in which the detector2 is connected to the bias electrode 4 and to a constant current source14. As discussed above, the voltage drop across the detector 2 changeswith incident radiation 12 and these voltage changes are coupled toamplifier 10 through capacitor 8.

A constant voltage mode of operation is shown in FIG. 1C. In this modeof operation, radiation induced resistance changes in the detector 2 arequantified by measuring the current flowing through the detector 2. Thisis typically accomplished by direct coupling of the detector 2 to theinput of a current-to-voltage converter, often identified as a"transimpedance amplifier". The transimpedance amplifier shown in FIG.1C includes operational amplifier 16 having one input terminal groundedand the other input terminal connected to the detector 2. Feedbackresistor 18 extends between the output terminal of operational amplifier16 and its input terminal receiving the output of the detector 2. Thedetector 2 is also connected to the bias electrode 4.

The bias voltage applied to a photoconductive detector also causescurrent to flow in the absence of incident radiation. This current,referred to as the "dark current", is usually large when compared to thecurrent changes resulting from incident radiation. The detection of thesmall, radiation related signal, which is added to the large, darksignal, is often difficult. A typical arrangement for detecting theradiation related signal is shown in FIG. 2. Radiation from a radiationsource 20, which could be a source of infrared radiation, passes throughand is modulated by a modulator 22, such as a rotating slotted chopperdisk, and then impinges upon a photoconductive detector 24. Thearrangement shown in FIG. 2 is a constant voltage mode of operation inwhich a DC bias voltage is applied to the detector 24 from bias voltagesource 26 and the output of the detector 24 is passed through atransimpedance amplifier 28 as discussed above in connection with FIG.1C. The detector 24 converts the modulated radiation 30 into anelectrical signal and the transimpedance amplifier 28 amplifies both theAC signal associated with the radiation 30 and the associated darksignal. Modulated detection, or preferably synchronous detection, isused to overcome the problem associated with the dark signal. The ACcomponent of the signal generated by the transimpedance amplifier 28 isseparated from the DC dark signal by coupling capacitor 32. Theresulting AC component from the coupling capacitor 32 can be rectifiedsynchronously with an external reference (not shown) derived from themodulator 22. The rectified signal from coupling capacitor 32 is passedthrough low pass filter 34, which can also include amplification, toreduce noise. The filtered signal is then digitized by ananalog-to-digital converter 36 and the resultant digital signal can beused as desired.

It is common to use a plurality of photoconductive detector elements inthe form of a linear array to measure radiation across a spectrum ofwavelengths. Each detector element is responsive to and detects aparticular wavelength, or band of wavelengths, of radiation. As shown inFIG. 3, a plurality of elongated photoconductive detector elements40-46, also referred to as pixels, is shown mounted on a commonsubstrate 48. Regardless of the type of biasing arrangement used, eachelement or pixel of the array will have one terminal, referred to as theinput terminal, connected to a common electrode associated with thebiasing source and another terminal, referred to as the output terminal,connected to the measuring scheme, which also provides a bias returnpath. As a result, the output terminals of the detector elementsconstitute a majority of the wiring and packaging feed-throughs from thesubstrate to the remainder of the system in which the substrate havingthe linear detector array is mounted. Rather than distribute the biassource along only one side of the detector array, i.e., with all of theinput terminals of the detector elements aligned along one side of thesubstrate and with all of the output terminals of the detector elementsaligned along the other side of the substrate, it is typical todistribute the input and output terminals of the detector elementsevenly on both sides of the linear array structure as shown in FIG. 3.This configuration permits the design to be arranged such that thepackaging is symmetrical and the interconnections are simplified. As canbe seen in FIG. 3, the measurement devices, here transimpedanceamplifiers 50-56, connected to the output terminals of detector elements40-46, respectively, are evenly distributed on opposite sides of thesubstrate 48, rather than all being positioned on one side. Typically,odd detector elements in the array have their output terminals on oneside of the substrate 48 while even detector elements in the array havetheir output terminal on the other side of the substrate 48. Such aconfiguration is often referred to as an "interdigitated" or"interlaced" arrangement. The arrangement shown in FIG. 3 is a constantvoltage mode of operation, including a bias voltage source 58 supplyinga bias voltage V_(b) to the input terminal of each detector element40-46 through a single bias electrode 60 on the substrate 48, andincluding a transimpedance amplifier 50-56 connected to the outputterminal of each detector element 40-46. The single bias electrode 60extends on the substrate along the length of the array and on each sideof the detector elements. However, other biasing arrangements can alsobe utilized.

The interdigitated design shown in FIG. 3 results in potentialdifferences between neighboring detector elements. This is shown in FIG.3 where, for example, detector element 40 has the bias voltage V_(b)applied to its input terminal while the immediately adjacent detectorelement 41 has a zero voltage at its output terminal which isimmediately adjacent the input terminal of detector element 40. Theopposite situation is shown at the opposite ends of detector elements 40and 41 where the bias voltage V_(b) is applied to the input terminal ofdetector element 41 which is adjacent the output terminal of detectorelement 40 having a zero voltage thereupon. The difference in voltagebetween the ends of adjacent detector elements 40 and 41 is the biasvoltage V_(b). In the typical microcircuit arrangements in which theselinear detector arrays are configured, the distance between the adjacentdetector elements can be as small as 0.001 inch in a medium resolutionarray. Even a relatively low bias voltage can result in a highelectrical field between the adjacent ends of adjacent detectorelements. This high electrical field can encourage the migration ofsurface contaminants to these areas and create leakage currentstherebetween. This configuration results in an increase in noise andinstability and, with time, may lead to the failure of the array.

It is, therefore, an object of the present invention to minimize theseproblems of the prior art interdigitated linear arrays and minimize oreliminate the potential difference and resulting electric field betweenadjacent detector elements in the array while preserving the small,compact design of a microcircuit arrangement.

SUMMARY OF THE INVENTION

Accordingly, we have developed a detector which includes a plurality ofseparate detector elements arranged on a substrate in a linear array.Each detector element has an input terminal and an output terminal. Eachdetector element is positioned on the substrate such that the inputterminal of each detector element is adjacent the output terminal of adetector element immediately adjacent thereto and the output terminal ofeach detector element is adjacent the input terminal of a detectorelement immediately adjacent thereto. A bias source is connected to theinput terminal of each detector element and a measurement device isconnected to the output terminal of each detector element. The biassource includes a first bias connected to the input terminals of a firstset of detector elements including alternating detector elements. Thefirst bias is also connected through an associated measurement device tothe output terminals of a second set of detector elements including theremaining detector elements. The bias source also includes a second biasdifferent from the first bias and connected through an associatedmeasurement device to the output terminals of the first set of detectorelements. The second bias is also connected to the input terminals ofthe second set of detector elements.

The detector can include a first bias electrode on the substrate whichextends along the length of the linear array on one side thereof and isconnected to the immediately adjacent input terminals of the first setof detector elements. The detector can also include a second biaselectrode on the substrate which extends along the length of the lineararray on the other side thereof and is connected to the immediatelyadjacent input terminals of the second set of detector elements. Thefirst bias can include a first bias voltage connected to the first biaselectrode and to the measurement devices for the second set of detectorelements. The second bias can include a second bias voltage connected tothe second bias electrode and to the measurement devices for the firstset of detector elements. The first and second bias voltages arenon-zero and are different from each other. In a preferred embodiment,the first bias voltage is a positive DC bias voltage and the second biasvoltage is the negative of the first bias voltage.

The measurement devices attached to each detector element can betransimpedance amplifiers including an operational amplifier and anegative feedback loop. In this arrangement, the first bias is suppliedto the operational amplifiers in the transimpedance amplifiersassociated with the second set of detector elements and is supplied tothe output terminals thereof through the negative feedback loopassociated with the respective operational amplifier. In thisarrangement, the second bias is supplied to the operational amplifiersin the transimpedance amplifiers associated with the first set ofdetector elements and is supplied to the output terminals thereofthrough the negative feedback loop associated with the respectiveoperational amplifier. The detector elements are preferablyphotoconductive detectors, such as detectors operating in the infraredrange.

We have also developed a method of testing a detector including aplurality of detector elements arranged on a substrate in a linear arrayand having a first bias electrode along one side of the linear array andconnected to the input terminals of alternating of the detector elementsand a second bias electrode extending along the opposite side of thelinear array and connected to the input terminals of the remainingdetector elements. The method uses the split bias arrangement discussedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are circuit diagrams showing, respectively, a voltagedivider, a constant current and a constant voltage bias for aphotoconductive detector element;

FIG. 2 is a circuit diagram of a detector system using the constantvoltage bias circuit shown in FIG. 1C above;

FIG. 3 is a circuit diagram showing a prior art interdigitated lineardetector array using the constant voltage bias circuit shown in FIG. 1C;and

FIG. 4 is a circuit diagram showing an interdigitated linear detectorarray using a split bias in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a bias arrangement for an interdigitatedlinear array of photoconductive detector elements is shown in FIG. 4.Similar to FIG. 3 discussed above, a plurality of detector elements70-76 is arranged on a substrate 78 in a linear array. Each detectorelement has an input terminal and an output terminal. Each detectorelement is positioned on the substrate 78 such that the input terminalof one detector element is adjacent the output terminal of a detectorelement immediately adjacent thereto and the output terminal of that onedetector element is adjacent the input terminal of a detector elementimmediately adjacent thereto. For example, the detector element 70 shownin FIG. 4 has its input terminal 70a immediately adjacent the outputterminal 71b of detector element 71 located immediately therebeneath.Similarly, detector element 71 has its input terminal 71a immediatelybeneath and adjacent the output terminal 70b of detector element 70located immediately thereabove. FIG. 4 also shows that the outputterminal of each detector element 70-76 is connected to a measurementdevice, here shown as transimpedance amplifiers 80-86 as discussed abovein connection with FIGS. 1C, 2 and 3. Although only seven detectorelements 70-76 are shown on the substrate 78 in FIG. 4, it is to beunderstood that a large number of detector elements and associatedmeasurement devices are typically utilized.

In accordance with the present invention, a first bias voltage V_(b)from bias voltage source 88 is connected to the input terminal of eachodd detector element 70, 72, 74, 76 in the linear array by means of afirst bias electrode 90 on the substrate 78. A second bias voltageNV_(b) from a second or negative bias voltage source 92 is connected tothe input terminal of the remaining or even detector elements 71, 73, 75in the linear array by means of a second bias electrode 94 on thesubstrate 78. The first bias electrode 90 extends along the side of thesubstrate 78 adjacent the input terminals of the odd detector elements70, 72, 74, 76 while the second bias electrode 94 is separate from thefirst bias electrode 90 and extends along the opposite side of thesubstrate 78 and adjacent the input terminals of the even detectorelements 71, 73, 75. The first bias voltage V_(b) is also supplied tothe positive input terminals of the operational amplifiers in thetransimpedance amplifiers 81, 83, 85 connected to the outputs of theeven detector elements 71, 73, 75. Similarly, the second bias voltageNV_(b) is connected to the positive input terminals of the operationalamplifiers in the transimpedance amplifiers 80, 82, 84, 86 connected tothe output terminals of the odd detector elements 70, 72, 74, 76. Thefirst bias voltage V_(b) or second bias voltage NV_(b), respectively, istransferred through a feedback resistor to the negative input terminalof an operational amplifier and supplied to the output terminals of thedetector elements associated therewith. In this manner, the input andoutput terminals of each detector element adjacent to each other alongone side of the detector substrate will have the same voltage appliedthereto and the potential difference between immediately adjacentelements will be zero.

As shown in FIG. 4, the first bias voltage V_(b) is applied through thefirst bias electrode 90 to the input terminal 70a of detector element70. Through negative feedback of the first bias voltage V_(b) throughtransimpedance amplifier 81 connected to the output terminal 71b ofadjacent detector element 71, the first bias voltage V_(b) also appearsat the output terminal 71_(b) of detector element 71. The voltage dropbetween the adjacent terminals 70a and 71b of detector elements 70 and71 near the first bias electrode 90 is V_(b) --V_(b) or zero as shown inFIG. 4. Similarly, the second bias voltage NV_(b) is applied through thesecond bias electrode 94 to the input terminal 71a of detector element71 and is applied through transimpedance amplifier 80 to the outputterminal 70_(b) of detector element 70. The voltage drop betweenadjacent terminals 70b and 71a of detector elements 70 and 71 near thesecond bias electrode 94 is NV_(b) --NV_(b) or zero as shown in FIG. 4.Similar voltages appear on the adjacent terminals of the remainingdetector elements on the substrate 78.

This split biasing scheme virtually eliminates the potential differencebetween the input and output terminals of neighboring detector elements.Since the bias across each detector element is the difference betweenthe first and second bias voltages, these bias voltages cannot beidentical. By providing the first bias voltage of a particular potentialand then providing the second bias voltage as the negative of the firstbias voltage as shown in FIG. 4, the circuit can be balanced withoutadditional complications. The bias voltages cannot be zero and cannot beof a higher potential than the supply voltage to the operationalamplifiers in the transimpedance amplifiers. As shown in FIG. 4, theoperational amplifiers in transimpedance amplifiers 80, 82, 84, 86associated with the odd detector elements 70, 72, 74, 76 are suppliedwith zero volts on their positive rail and with -30 volts on theirnegative rail. The operational amplifiers in transimpedance amplifiers81, 83, 85 associated with the even detector elements 71, 73, 75 aresupplied with +30 volts on their positive rail and with zero volts ontheir negative rail. In a typical prior art arrangement, the operationalamplifiers would be supplied with +15 volts on their positive rail and-15 volts on their negative rail. The voltages shown in FIG. 4 allow forthe use of higher bias voltages to the detector elements and an increasein signal compliance range.

When a voltage divider arrangement is used for the biasing andmeasurement of radiation related signals, each load resistor must beadjusted to the particular detector element to achieve potentialequalization since the resistance of each detector element may bedifferent. For the same reason, the current source in a constant currentbias arrangement must be matched to the resistance of the particulardetector element. The constant voltage bias arrangement shown in FIG. 4is preferred in that it does not require individual adjustment for eachdetector element since the transimpedance amplifier supplies the otherbias voltage to the output terminals of the detector elements throughnegative feedback.

In addition to improving array reliability, the biasing arrangementshown in FIG. 4 permits simple testing of the linear array forcontamination. When the substrate 78 is disconnected from itselectronics and bias voltage sources, application of a DC potentialbetween the two bias electrodes 90, 94 creates a potential betweenneighboring or adjacent array elements. This can be measured by means ofa simple ohmmeter. In the absence of contaminants, the resistanceobserved between those electrodes will be nearly infinity. The presenceof contaminants on the array will result in a much lower resistance ascurrent starts to flow between neighboring detector elements. Thissimple test merely determines whether contaminants are present or not.

A more refined test can be performed with the array electronics and biasvoltage sources attached to the substrate 78. The effect ofinter-element potential can be measured by switching the bias voltagesources between the two bias electrodes 90, 94. The second bias voltageNV_(b) is applied to the first bias electrode 90 normally carrying thefirst bias voltage V_(b), while the first bias voltage V_(b) isconnected to the second bias electrode 94 normally carrying the secondbias voltage NV_(b). The bias voltages supplied to the transimpedanceamplifiers 80-86 as shown in FIG. 4 remain the same. By analyzing, forexample, the first two detector elements 70 and 71 in FIG. 4, it can beappreciated that the second bias voltage NV_(b) would then be applied toboth sides of detector element 70 while the first bias voltage V_(b)would be applied to both sides of detector element 71. Because of thezero potential across each detector element, no current should flow toeither transimpedance amplifier 80 or 81 in absence of contaminants. Ifcontaminants are present, the potential difference of V_(b) --NV_(b)applied between two adjacent detector elements, for example betweeninput terminal 70a of detector element 70 and output terminal 71b ofdetector element 71 or between input terminal 71a of detector element 71and output terminal 70b of detector element 70, will cause current toflow to either associated transimpedance amplifier or to both. Themagnitude of these currents and their ratio allows one to assess thedegree of contamination and to estimate the location of thecontamination along the length of the associated detector elements.

Having described above the presently preferred embodiments of thepresent invention, it is to be understood that the invention may beotherwise embodied within the scope of the appended claims.

We claim:
 1. A detector comprising a plurality of separate detectorelements arranged on a substrate in a linear array, with each detectorelement having an input terminal and an output terminal, with eachdetector element positioned on the substrate such that the inputterminal of each detector element is adjacent the output terminal of adetector element immediately adjacent thereto and the output terminal ofeach detector element is adjacent the input terminal of a detectorelement immediately adjacent thereto, a bias source connected to theinput terminal of each detector element, and a measurement deviceconnected to the output terminal of each detector element, wherein thebias source includes a first bias connected to the input terminals of afirst set of detector elements including alternating detector elementsand connected through an associated measurement device to the outputterminals of a second set of detector elements including the remainingdetector elements, and wherein the bias source includes a second biasdifferent from the first bias and connected through an associatedmeasurement device to the output terminals of the first set of detectorelements and connected to the input terminals of the second set ofdetector elements.
 2. The detector of claim 1 further including a firstbias electrode on said substrate and extending along the length of saidlinear array on one side thereof and connected to the immediatelyadjacent input terminals of the first set of detector elements, and asecond bias electrode on said substrate and extending along the lengthof said linear array on the other side thereof and connected to theimmediately adjacent input terminals of the second set of detectorelements.
 3. The detector of claim 2 wherein the first bias includes afirst bias voltage connected to said first bias electrode and to themeasurement devices for the second set of detector elements and whereinthe second bias includes a second bias voltage connected to the secondbias electrode and to the measurement devices for the first set ofdetector elements, with the first and second bias voltages beingnon-zero and different from each other.
 4. The detector of claim 3wherein the first bias voltage is a positive DC bias voltage and thesecond bias voltage is the negative of the first bias voltage.
 5. Thedetector of claim 1 wherein the measurement device attached to eachdetector element is a transimpedance amplifier including an operationalamplifier and a negative feedback loop, wherein the first bias issupplied to each operational amplifier in the transimpedance amplifierassociated with each of the second set of detector elements and issupplied to the output terminals thereof through the negative feedbackloop associated with the respective operational amplifier, and whereinthe second bias is supplied to each operational amplifier in thetransimpedance amplifier associated with the first set of detectorelements and is supplied to the output terminals thereof through thenegative feedback loop associated with the respective operationalamplifier.
 6. The detector of claim 5 wherein the detector elements arephotoconductive detectors.
 7. The detector of claim 6 wherein thephotoconductive detectors operate in the infrared range.
 8. The detectorof claim 6 wherein the detector elements are formed from the groupconsisting of PbS or PbSe.
 9. The detector of claim 5 wherein themagnitude of the bias voltage is less than the magnitude of a supplyvoltage provided to the operational amplifiers in the transimpedanceamplifiers.
 10. A detector comprising a plurality of separate detectorelements arranged on a substrate in a linear array, with each detectorelement having an input terminal and an output terminal, with eachdetector element positioned on the substrate such that the inputterminal of each detector element is adjacent the output terminal of adetector element immediately adjacent thereto and the output terminal ofeach detector element is adjacent the input terminal of a detectorelement immediately adjacent thereto, a first bias electrode on saidsubstrate and extending along the length of said linear array on oneside thereof and connected to the immediately adjacent input terminalsof a first set of alternating detector elements, and a second biaselectrode on said substrate, separate from the first bias electrode, andextending along the length of said linear array on the other sidethereof and connected to the immediately adjacent input terminals of asecond set of the remaining alternating detector elements.
 11. Thedetector of claim 10 wherein the detector elements are photoconductivedetectors.
 12. The detector of claim 11 wherein the photoconductivedetectors operate in the infrared range.
 13. The detector of claim 11wherein the detector elements are formed from the group consisting ofPbS or PbSe.
 14. The method of testing the detector set forth in claim10 comprising the steps of:a) applying a DC voltage between the firstbias electrode and the second bias electrode, and b) measuring theresistance between the adjacent terminals of adjacent detector elements.15. The method of testing the detector set forth in claim 10 comprisingthe steps of:a) connecting a transimpedance amplifier to the outputterminal of each detector element, b) applying a first DC bias voltageto the second bias electrode and, through the associated transimpedanceamplifier, to the output terminals of the second set of detectorelements, c) applying a second DC bias voltage, different from the firstDC bias voltage, to the first bias electrode and, through the associatedtransimpedance amplifier, to the output terminals of the first set ofdetector elements, and d) measuring the currents flowing to thetransimpedance amplifiers.
 16. The detector of claim 2, wherein themeasurement device attached to each detector element is a transimpedanceamplifier including an operational amplifier and a negative feedbackloop, wherein the first bias is supplied to each operational amplifierin the transimpedance amplifier associated with each of the second setof detector elements and is supplied to the output terminals thereofthrough the negative feedback loop associated with the respectiveoperational amplifier, and wherein the second bias is supplied to eachoperational amplifier in the transimpedance amplifier associated withthe first set of detector elements and is supplied to the outputterminals thereof through the negative feedback loop associated with therespective operational amplifier.
 17. The detector of claim 3, whereinthe measurement device attached to each detector element is atransimpedance amplifier including an operational amplifier and anegative feedback loop, wherein the first bias is supplied to eachoperational amplifier in the transimpedance amplifier associated witheach of the second set of detector elements and is supplied to theoutput terminals thereof through the negative feedback loop associatedwith the respective operational amplifier, and wherein the second biasis supplied to each operational amplifier in the transimpedanceamplifier associated with the first set of detector elements and issupplied to the output terminals thereof through the negative feedbackloop associated with the respective operational amplifier.
 18. Thedetector of claim 4, wherein the measurement device attached to eachdetector element is a transimpedance amplifier including an operationalamplifier and a negative feedback loop, wherein the first bias issupplied to each operational amplifier in the transimpedance amplifierassociated with each of the second set of detector elements and issupplied to the output terminals thereof through the negative feedbackloop associated with the respective operational amplifier, and whereinthe second bias is supplied to each operational amplifier in thetransimpedance amplifier associated with the first set of detectorelements and is supplied to the output terminals thereof through thenegative feedback loop associated with the respective operationalamplifier.